Non-invasive Stimulation of the Glymphatic System for Slowing Cognitive Decline

Inclusion Criteria - Healthy Participants : * Age 20-85 years * Willing and able to undergo all procedures * Free of any neurological (e.g., epilepsy, stroke, Parkinson's, brain tumors, moderate or severe traumatic brain injury), neuropsychiatric (e.g., schizophrenia, substance abuse, clinical depression), or developmental (e.g., autism, ADHD) or any other medical condition at the discretion of the PI and co-investigators Inclusion Criteria - mTBI : * Age 20-65 * Diagnosed with mild traumatic brain injury and be within 1-52 weeks post-injury * Willing and able to undergo all procedures Exclusion Criteria - Healthy Participants: * Contraindication to MRI * Known allergies to common electrode materials Exclusion Criteria - mTBI: * Contraindications for MRI * Known allergies to common electrode materials * Candidates currently on cardiovascular medications which, in the opinion of the investigators, would introduce a confounding factor to glymphatic system function (i.e. ACE-inhibitors, angiotensin receptor blockers, calcium channel blockers) * Candidates who have chronic infectious diseases (e.g. hepatitis, HIV, TB) * Candidates diagnosed with small vessel disease, vascular associated dementia (e.g. VCID), or renal disease * Candidates retinopathies or neuropathies that, in the opinion of the investigators, would prevent them from being able to complete the study or would introduce a confounding factor to glymphatic system function * Candidates with neurological disorders other than those attributed to their primary diagnosis (e.g. MS, PD, ALS, AD or other dementia, uncontrolled pain) * Candidates who have had a penetrating injury, craniotomy (with the exception of a burr hole (trephination) for resolution of acute subdural hematoma), or refractory subdural hematoma * Candidates with a history of seizures (except those in the acute or post-acute phases and are controlled) * Candidates who experienced a loss of consciousness greater than 24 hours as a result of their TBI * Candidates who, in the opinion of the investigators, are unable to feel a sensory referred sensation from the stimulation or successfully complete the electrode placement and testing fNIRs